As the main terminus of the spinothalamic tract (STT) the area of the principle sensory nucleus of human thalamus (ventrocaudal -Vc) has often been implicated in nociception. We have studied Vc during the physiologic exploration which precedes stereotactic procedures for pain and movement disorders. Our previous studies have shown that cells in the area of Vc are responsive to stimuli which were described as painful by the patients tested. These cells may encode pain since stimulation at their recording sites often evoked the sensation of pain. However, stimulation in the region of Vc evoked non-painful thermal sensations more frequently than pain suggesting that a population of cells in this area may encode non- painful thermal sensations. Preliminary studies indicate that stimulation in Vc evoked pain more frequently in patients with chronic pain and hyperalgesia than in patients with movement disorders. The percentage of sites were stimulation evoked pain in patients with chronic pain was greater than that in movement disorder patients by an amount equal to the decrease in the percentage of sites where non-painful thermal sensations were evoked. These results suggest that stimulation in patients with chronic pain may evoke the sensation of pain at sites where the non- painful thermal sensations would normally be evoked. We now propose to study whether cells in Vc mediate both the non-painful thermal sensations evoked by stimulation in patients with movement disorders and the pain sensations evoked by stimulation in patients with chronic pain. The response of cells at the recording site to graded mechanical and thermal stimuli into the painful range will be studied and compared with simultaneous psychophysical ratings of stimulus intensity and pain. For example, the neural response to thermal and mechanical stimuli will be compared with psychophysical ratings of temperature and pain and sensations evoked by stimulation at the recording site to assess whether cellular activity is better related to the sensation of temperature of pain. The stimulus response functions for neural activity in patients with movement disorders will be compared with those in patients with chronic pain. This proposal has the potential to demonstrate thalamic cellular activity normally correlated with thermal sensations and pain in patients with movement disorders. Changes in the psychophysical response to sensory stimuli in patients with chronic pain may be shown to parallel changes in the cellular response to the same stimuli.